1. Field of the Invention
The present invention relates to an optical fiber having a grating and its manufacturing method.
2. Description of the Related Art
For an optical fiber having a grating, a single mode fiber, a high Ge dope dispersion shift fiber and the like are used. In such an optical fiber, for example, as shown in FIG. 11A, the optical fiber comprises a core 2 and a cladding 3 formed outside the core 2. At one part of at least the core 2 is formed a grating 2a (a portion shown by oblique lines in FIG. 11A) having a different refractive index distribution from the refractive index distribution of other portions. That is, a so-called fiber grating is performed (simply referred to as "grating processing" below) for the optical fiber. As shown in FIG. 11B, the optical fiber has a refractive index which is longitudinally changed. The grating 2a functions mainly as an optical filter and the like for reflecting a light alone having a specific wavelength.
FIGS. 12 and 13 show a structure and a manufacturing method of the optical fiber having the grating relating to the present invention.
In the first place, as shown in FIG. 12, an optical fiber 5 is so constructed that a coating layer 4 consisting of an ultraviolet-curing resin and the like is formed outside a bare optical filter 1. In the optical fiber 5, the coating layer 4 is partly removed, so that an exposed portion la is formed. A desired portion of the exposed bare optical fiber 1 is grating-processed.
Relating to the grating processing, FIG. 13 shows an example by means of a holographic method. In FIG. 13, the grating-processed portion is shown by the oblique lines.
As shown in FIG. 13, for example, an ultraviolet light UV is split by a beam splitter 8, so that each split ultraviolet light UV is reflected by a mirror 9. Thus, an interference band pattern of the ultraviolet light UV is formed. The exposed bare optical fiber 1 is irradiated on a surface with the ultraviolet light UV.
In such a manner, the exposed bare optical fiber 1 is irradiated with the ultraviolet light UV. At this time, for example, if the bare optical fiber 1 is a Ge dope optical fiber, the refractive index is increased at a core portion of the exposed bare optical fiber 1 in accordance with an irradiation strength of the ultraviolet light UV.
In case of the bare optical fiber 1 shown in FIG. 13, there are the portion having the strong irradiation strength of the ultraviolet light UV and the portion having the weak one. Thus, in the bare optical fiber 1, the refractive index is greatly increased at the portion having the strong irradiation strength of the ultraviolet light UV (shown by a thicker oblique line portion A). The refractive index is little increased at the portion having the weak irradiation strength of the ultraviolet light UV (shown by a thinner oblique line portion B).
As a result, as shown by the oblique line portions A and B in FIG. 13, the bare optical fiber 1 has the portion where the refractive index distribution is longitudinally periodically changed. Such a characteristic is obtained that the light alone having the specific wavelength is reflected at the oblique line portion where the refractive index is changed.
In such a manner, the grating processing is performed. Subsequently, in the optical fiber 5, as shown in FIG. 14, the exposed portion la is accommodated in a sleeve 7 which is made of a rigid material such as a rigid plastic and a metal so as not to irradiate the exposed bare optical fiber 1 with the excessive ultraviolet light UV and so as to protect the surface from an external force. Furthermore, the optical fiber 5 and the sleeve 7 are adhered to both ends of the coating layer 4 by an adhesive 6 so that they may be integrated with each other.
In such a manner, the optical fiber 5 having the grating is manufactured.
However, the optical fiber 5 having the structure shown in FIG. 14 is difficult to be bent at the sleeve 7, since the sleeve 7 is rigid. Thus, for example, when a bending load is applied to the optical fiber 5, a stress is concentrated near both of the ends of the sleeve 7. Accordingly, there is arisen such a problem that a light transmission loss is increased and a rupture easily occurs.
For example, there are prepared the ten optical fibers 5 having the sleeve 7 of a 5-mm outer diameter and a 2-cm length. The optical fibers 5 are held for two months in a state that they are coiled around a mandrel having the outer diameter of 30 mm at a tension of 20 g-weight. As a result, nine of the ten optical fibers 5 rupture at the portion near both of the ends of the sleeve 7.
Moreover, the optical fiber 5 has been conventionally examined so that it may be incorporated in a transmitter of, for example, an optical repeater or the like in order to use it therein. Recently, there are user's increasing needs that the optical fiber is arranged in, for example, an optical fiber cable so as to be used.
However, according to the optical fiber 5 having the structure shown in FIG. 14, there is structurally an excessive space between the exposed portion la and the sleeve 7 of the bare optical fiber 1. Accordingly, the sleeve 7 has the thicker diameter. Thus, there is arisen the problem that the optical fiber 5 having the grating cannot be arranged in the optical fiber cable whose accommodating space is small.
On the other hand, as described above, when the optical fiber 5 having the grating is manufactured, the coating layer 4 formed on the bare optical fiber 1 is partly removed for irradiating the ultraviolet light. Accordingly, in the exposed bare optical fiber 1 due to a removal of the coating layer 4, the cladding is being exposed to an air having the smaller refractive index than the refractive index of the cladding. Thus, according to the bare optical fiber 1 wherein the grating is formed, at the exposed portion la exposed to the air, a cladding-mode light is leaked from the core to the cladding, so that the leaked light is combined to a waveguide mode which passes through the core.
As a result, in a transmission power of a transmitted light which passes through the optical fiber 5, a ripple occurs, due to the cladding mode, at a shorter wavelength side than a Bragg wavelength. The ripple enters, as a noise, into a light signal transmitted in the optical fiber 5, so that a reliability of an optical communication is reduced.